1. Field of the Invention
The present invention relates generally to free space optical communications systems. More particularly, the present invention is directed towards power level management in free space optical communication systems.
2. Description of the Related Art
Free space optical communication systems are of interest as a technique to provide high data rate communications. In a free space optical communication system, a transmitter transmits data, typically at infrared wavelengths, to a line-of-sight receiver. Applications of free space optical communications systems include, for example, providing links to, from or between aircraft, spacecraft and ground vehicles and stations (including those that are water-based), as well as delivering fiber optical speed and services to sites, especially those that do not have direct fiber access to optical fiber networks.
Referring to FIG. 1A, a free space optical transceiver 10 at a first location generates a free space optical beam propagating to a corresponding transceiver 10 at a second location along an optical path 12. However, wind, thermal effects, and turbulent air motion causes a variation in air density along the length of the optical path. At any point in time, there will be regions of differing air density proximate the optical path 12. Moreover, over time periods of a fraction of a second, the air density may change along different regions of the optical path 12.
Referring to FIG. 1B, the regions of varying air density are sometime referred to as turbulence cells 2 or “turbules.” The turbulence cells grow, shrink, and move around at rates of about 100 Hz. The variations in refractive index associated with turbulence cells in the atmosphere may be approximated in a geometrical optics approximation as a sequence of lenses along the optical path that cause beam wander, since portions of the beam will be directed off-axis by the lensing effect of the turbulence cells. Additionally, the effect of turbulence cells may be modeled as causing random variation in the wavefront. This leads to interference-induced phase and amplitude variations along the wavefront at the receiver, producing a speckle pattern across a receiver called scintillation.
Scintillation and weather can cause large variations in received power levels and variations in signal-to-noise ratios. This is one of the factors that has previously limited the applications of free space optics, as a substitute for conventional fiber optic links and otherwise.
Thus, there is a need for a free space optical communication system with improved control of beam characteristics and power levels.